Sequential prospective studies of lymphocyte function, chemotactic activity of neutrophils, neutrophil antibacterial function, monocyte antibacterial function, skin antigen reactivity, serum inhibitors and serum opsonic activity and components are being measured in surgical patients with a high risk of infection, namely those post-transplant, severely burned, severely injured and already infected. Correlation with the clinical course and bacteriologic data will identify predisposing immunologic factors and the effect of infection on these variables. The effects of protein nutrition on these same immunologic variables is being studied in both animals and man. The potential benefit of plasma therapy in repair of opsonic defects is also being evaluated in both animals and man. Opsonic requirements for various bacteria are being studied and correlations being made with pathogenicity.